1. Field of the Invention
The present invention relates to a circuit for the stepwise variation of an operating voltage of a deflection amplifier of a magnetodynamic deflection system for a charged-particle beam in dependence upon a required deflection speed which is particularly useful in a television monitor which serves as the display equipment of a charged-particle beam apparatus.
2. Description of the Prior Art
In magnetodynamic deflection systems, a charged-particle beam is deflected by the magnetic field of a coil. The magnitude of this deflection is proportional to the current which flows through the coil; the deflection velocity, therefore, depends upon the rate of change of the current flow. At velocities at which the inductive reactance of the coil predominates, the rate of change of the current flow is proportional, in the current range utilized, to the voltage applied to the deflection coil. The faster the beam is to be deflected, the higher the voltage must be at the deflection coil. This also requires a high supply voltage at the deflection amplifier. The higher this supply voltage, however, the higher the power dissipation, for example, in the collector resistors of the deflection amplifier. If the deflection amplifier were to be continuously operated at the highest supply voltage required for maximum deflection velocity, a power dissipation loss would result which would be intolerably high for the application.
In television receivers, for example, in which there are only two different deflection velocities, namely, that for the forward sweep and that for the substantially faster flyback, power loss is reduced through the use of a power-reducing resonance system which is tuned to the operating frequency of the flyback. In television monitors used as display devices in scanning charged-particle beam microscopes, however, another operating range with frame times between 20 ms and hundreds or thousands of seconds must be covered and it should be possible to operate in modes such as raster scan and random positioning (vector writing). For such a broad frequency spectrum of deflection velocities, a resonant system is not usable for power reduction.
In a magnetodynamic deflection system presently known in the art, the high operating voltage required for rapid deflection is applied to the deflection amplifier only during brief, defined periods of time and otherwise remains at a lower, normal value. See U.S. Pat. No. 3,887,829. The power dissipation loss is, therefore, reduced to a small residual amount. The brief period during which the operating voltage is high is, therefore, limited for raster scan to the line flyback, and in the case of random positioning, to writing particularly rapid (long) vectors. In the magnetodynamic deflection system described in the foregoing patent, two separate power supply sources are used, one of which furnishes a normal supply voltage for slow deflection and the other one of which furnishes a high supply voltage for rapid deflection.